OUTLINES OF GEOLOGY. 



7? 



ORDER V. TRACHYTIC. 



178. The rocks of the Trachytic order are : 

 §|||||1. Trachyte, composed of vitrous felspar ; 



2. Domite, felspar having an earthy aspect ; 



3. Argillophyre, a porphyritic rock, whose paste is a 

 decomposing felspar ; 



4. Klinkstone ) a compact rock whose constituents are 

 the same as felspar ; 



5. Perlite ; felspar having a pearly lustre, and which is 

 sometimes porphyritic. 



179. The distinctive character of these rocks is, that the 

 felspar of which they are composed appears to have been 

 modified by heat. 



180. The rocks of this order form mountains, among which 

 are some of the most elevated on the surface of the earth. 

 Among these may be cited Chimborazo. In their general 

 form, and from the traces of craters, these mountains resem- 

 ble those which are decidedly volcanic. 



181. Brogniart fixes the lower limit of this order at the 

 chalk, and it may be traced into the higher formations of the 

 superior order. It thus appears cotemporaneous with the 

 newer trapp and older volcanic formations. 



182. This order has not been discovered within the limits 

 of the United States. 



183. We might add to the order of ancient unstratified 



rocks, the volcanic, which has for its lower limit the higher 



limit of the trapp rocks. These two orders indeed change 



into each other by insensible degrees, and in the neighbour^ 



7 * 



